Process for the preparation of non-toxic anthrax vaccine

ABSTRACT

Anthrax toxin, comprising of protective antigen (PA), lethal factor (LF) and edema factor (EF) is a major virulent factor of  B. anthracis . Protective antigen, PA is the main component of all the vaccines against anthrax. The protective efficacy of PA is greatly increased if small quantities of LF of EF are incorporated into the vaccines. An ideal vaccine against anthrax should contain PA, LF and EF together, but this combination would be toxic. Therefore, the biologically inactive mutant preparations of PA, LF and EF may be used together for better immunoprotection. The present invention describes the method for generation of recombinant vaccine against anthrax, comprising of non-toxic, mutant anthrax toxin proteins. The procedure involves site-directed mutagenesis of the native genes of the toxin proteins, the expression and purification of the mutant proteins and finally characterization of these proteins.

FIELD OF THE INVENTION

This invention relates to a Recombinant DNA construct and a process for the preparation of a nontoxic anthrax vaccine.

BACKGROUND OF THE INVENTION

Anthrax, a zoonotic disease is caused by gram-positive, sporulating bacteria, Bacillus anthracis. Humans are accidental hosts through food of animal origin, animal products and contamination of the environment with Bacillus anthracis (Brachman P. S., 1970, Aninals. N.Y. Acad. Sci. 174, 577-582). Anthrax is one of the oldest known bacterial diseases and occurs in most parts of the world including India. The major virulent factors of B. anthracis include poly-D-glutamic acid capsule and a three-component anthrax toxin complex. Anthrax toxin (Leppla S. H., 1991, In Source Book of Bacterial protein toxins, pp 277-302.), comprising of protective antigen PA(83 kDa), lethal factor (LF-(90 kDa) and edema factor (EF-(89 kDa) is a major virulent factor of B. anthracis. LF/EF, the catalytic moieties of this complex require PA to enter the cell cytosol. PA in combination with LF (called the lethal toxin), causes death in experimental animals (Smith H. and Keppie J., 1954, Nature, 173, 869-870). PA in combination with EF (called the edema toxin) causes edema in the skin of the experimental animals (Stanley J. L. and Smith H., 1961, J. Gen Microbiol., 26, 49-66). PA is the receptor-binding moiety that facilitates the translocation of the catalytic moieties, LF and EF, into the target cells. After translocation into the cell, LF, a metalloprotease causes cleavage of certain Mitogen so activated protein kinase kinases (MAPKKs) resulting in inactivation of signal transduction pathways (Duesbery N. S., et. al., 1998, Science, 280. 734-737). On the other hand, EF, upon entering the cells, gets activated by calmodulin to cause massive increase in intracellular cAMP levels (Leppla S. H., 1982, Proc. Natl. Acad. Sci. USA., 79, 3162-3166).

The first step of the intoxication process is the binding of PA to the cell surface receptors (Bradley K. A. et al, 2001, Nature, 414 , 225-229). After binding to the receptors on the cell surface, PA gets nicked by cell surface proteases to yield a 63-kDa fragment (Klimpel el R. K., et. al., 1992, Proc. Natl. Acad. Sci. USA., 89, 10277-10281) which oligomerizes and binds to LF/EF (Milne J. C., et. al., 1994, J. Biol. Chem., 269, 20607-20612). Binding of LF/EF is competitive. The whole complex then undergoes receptor-mediated endocytosis. Acidification of the endosonie (Friedlander A. M., 1986, J. Biol. Chem.. 261 , 7123-7126) results in the insertion of the PA-oligomer into the endosomal membrane to form pores (Milne J. C. and Collier R. J., 1993 Mol. Microbiol., 10, 647-653) through which LF/EF are translocated into the cell cytosol.

PA has four domains that are organized primarily into antiparallel-beta sheets with only a few short helices of less than four turns (Petosa C., et. al., 1997, Nature, 385, 833-838). Domain 1 is responsible for binding to LF/EF during the anthrax intoxication process. Domain 2 is dominated by a beta barrel and plays a role in membrane insertion and translocation. Domain 3 is the smallest domain and is important for oligomerization of PA and possibly also in the binding of PA to LF/EF. Domain 4 is the receptor-binding domain.

Crystal structure of LF, determined recently, shows that LF has 4 domains (Pannifer A. D., et al, 2001, Nature, 414, 229-233). Domain 1 is involved in binding to PA. This domain has significant homology to the N-terminal 1-250 residues of EF. In fact, most of the residues in this region are absolutely conserved.

Of all the three toxin proteins, —PA is the most immunogenic and is an essential component of the vaccine against anthrax (Gladstone G. P., 1946, Br. J. Exp. Pathol, 97, 349-418). It has been observed that the protective efficacy of PA is greatly increased if small quantities of LF or EF are incorporated into the vaccine (Pezard et. al., 1995, infect. Immun., 63, 1369-1372). However, this also happens to be the primary reason of toxigenicity and reactogenicity of the vaccines. Anthrax toxin (Leppla S. H., 1991, In Source Book (of Bacterial protein toxins, pp 277-302.), comprising of protective antigen (PA), lethal factor (LF) and edema factor (EF) is a major virulent factor of B. anthracis.

The currently used anthrax vaccine is derived from a non-capsulated, avirulent strain of the bacterium known as Sterne's strain (Sterne M., 1939, J. Vet. Sci. Anim. Ind, 13, 307-312). In Russia and China, the live spore vaccines based on Sterne strain are used. In UK the vaccine is alum precipitated culture filtrate of the Sterne strain while the US vaccine consists of an alhydrogel-adsorbed cell free culture filtrates of a non-capsulating, non proteolytic derived strain V770 isolated from bovine anthrax (Turnbull P. C. B, 1991, Vaccine, 9, 533-539). All these currently used anthrax vaccines, apart from being crude have undefined composition. They are reactogenic and do not provide protection against all natural strains of B. anthracis.

U.S. Pat. No. 2,017,606 describes the preparation of anthrax antigen by growing the bacilli with a suitable culture medium, separating the bacilli from the culture medium.

U.S. Pat. No. 2,151,364 describes a method of producing an anthrax vaccine which comprises preparing the suspension of anthrax spores, adding to the suspension a sterile solution containing alum.

RU patent 2,115,433 describes the method of production of anthrax vaccine, which comprises of living spores of non-capsulated strain of B. anthracis and protective antigen of B. anthracis.

WO patent 000252 describes a method of production of anthrax vaccine using non-toxic protective antigen from B. anthracis for use in inducing immune response, which is protective against anthrax.

The drawbacks in the above-mentioned patents are that all of them use Bacillus anthracis cultures/spores. Bacillus anthracis is an infectious organism and can not be handled without containment facilities. The vaccine prepared is contaminated with other toxic and non-toxic proteins from Bacillus anthracis resulting in a number of side effects and reactogenicity.

These vaccines also have a certain degree of residual virulence for certain species of domesticated and laboratory animals. The Sterne strain is toxigenic and is pathogenic at high doses. As a result it is considered unsafe and unsuitable for human use. This vaccine can cause undesirable side effects including necrosis at the site of inoculation.

Therefore there is a need to develop a second-generation anthrax vaccine which does not have side effects and has a well-defined composition.

The object of the present invention is to render the anthrax toxin non-toxic without affecting its immunogenicity, in order to develop a safe and effective anthrax vaccine.

To achieve said object, the present invention provides a recombinant DNA construct comprising an expression vector and a DNA fragment including genes for wild type Protective Antigen (PA) or wild type Lethal Factor (LF) or wild type Edema Factor (EF)

The present invention also provides a recombinant DNA construct comprising:

an expression vector and a DNA fragment including genes for mutant type Protective Antigen (PA) or mutant type Lethal Factor (LF) or mutant type Edema Factor (EF).

Said vector is a prokaryotic vector such as PQE 30 and said expression vector contains T5 promoter and 6X histidine tag.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Phe202.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Leu203.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Pro205.

The said DNA fragment is the gene for protective antigen with Alanine-substitution at residue Ile207.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residues Pro205, Trp226 and Phe236.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Phe552.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Ile574.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Phe552 and Phe554.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Ile562 and Ile574.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Leu566 and Ile574.

The DNA fragment is the gene for protective antigen ith Alanine-substitution at residue Phe552 and Phe554, Ile562, Leu566 and Ile574.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Phe427.

The DNA fragment is the gene for protective antigen with deletion of residue Asp 425.

The DNA fragment is the gene for protective antigen with deletion of residue Phe 427.

The DNA fragment is the gene for protective antigen With Alanine-substitution at residue Trp346.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Leu352.

The DNA fragment is the gene for protective antigen with Alanine-substitution at residue Trp346, Met350 and Leu352.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Tyr148.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Tyr149.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Ile151.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Lys153.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Asp187.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Phe190.

The DNA fragment is the gene for lethal factor with Alanine-substitution at residue Asp 187, Leu188, Leu 189 and Phe 190.

The DNA fragment is the gene for edema factor with Alanine-substitution at residue Tyr137.

The DNA fragment is the gene for edema factor with Alanine-substitution at residue Tyr138.

The DNA fragment is the gene for edema factor with Alanine-substitution at residue Ile140.

The DNA fragment is the gene for edema factor with Alanine-substitution at residue Lys142.

The protein encoded by said DNA fragment is expressed in a prokaryotic host. The said prokaryotic host is an E. coli strain.

A protein expressed by gene DNA fragment is wild type PA wild type LF, wild type EF and their mutagenised variants.

This invention further discloses a method for producing mutagenized anthrax toxin protein comprising:

-   -   mutagenizing PA LF & EF genes using different mutagenic primers         of the kind as herein defined for PCR reaction;     -   treating said mutant PCR product along with the native template         with a n enzyme to cleave the native template of said PCR         product;     -   transforming said mutant product in E. coli strain;     -   isolating the recombinant construct from transformed E. coli         strain and confirming the desired mutation;     -   transforming the confirmed mutant construct in appropriate E.         coli expression strain to express the mutant protein and     -   purifying the said expressed mutant protein.

The purification is carried Out using Ni-NTA chromatography and/or other chromatographic techniques.

The genes are cloned in PQE expression vector containing T5 promoter and 6X histidine tag.

The mutations were affected in the first domain of PA at residues 202, 203, 205. The mutations were affected in the third domain of PA at residues 552, 574 552+554, 562+574, 566+574, 552+554+562+566+574 resulting in mutant proteins that were defective in oligomerization. The mutations were affected in the second domain of PA at residues 425 & 427 of loop 4 of domain 2. These mutations impaired the translocation-ability of PA The mutations were affected in the second domain of PA at residues 346, 352 and 346+350+352 in loop 3 of domain 2 such that PA becomes biologically inactive. The mutations were affected in the 1^(st) domain of LF at residues 148, 149, 151, 153, 187, 190 and 187+188+189+190 impaired the binding of LF to PA. The mutations were affected in the 1^(st) 250 residues of EF.

An anthrax vaccine comprising an anthrax toxin protein is selected from wild type PA or wild type LF or wild type EF.

An anthrax vaccine comprising an anthrax toxin protein selected from mutant type PA or mutant type LF or mutant type EF or a combination thereof.

An anthrax vaccine comprising an anthrax toxin protein selected is a combination of any one selected from wild type PA or wild type LF or wild type EF with any one or more selected from mutant type PA or mutant type LF or mutant type EF.

A pharmaceutical composition comprises an effective amount of an anthrax toxin protein as claimed by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS:

The present invention will be further explained in detail below with reference to the drawings, wherein

FIG. 1 shows cloning of PA, LF and EF in a PQE 30 vector; and

FIG. 2 shows site directed mutagenesis using long PCR.

DETAILED DESCRIPTIOON OF THE INVENTION

An ideal vaccine against anthrax should contain PA, LF, EF together, but at the same time it should be non-toxic and safe. Purified recombinant proteins with defined composition may be used in the vaccine to minimize reactogenicity of the vaccine. Further, these anthrax toxin proteins may be rendered non-toxic by introducing mutations that affect the biological activity of the proteins without affecting their structure or immunogenicity. These non-toxic, mutant anthrax toxin proteins may be used together to create a safe, non-reactogenic and effective recombinant vaccine against anthrax. Thus, the prime objective of this invention was to create a process for making a safe and effective, second-generation vaccine against anthrax comprising of non-toxic anthrax toxin proteins that have been produced by site-directed mutagenesis of the different functionally important domains of the toxin proteins.

The inventors of this application have PCR amplified the genes for PA, LF and EF. They have cloned these genes in pQE30 expression vector (Gupta P., et. al., 1998, Infect. Immun., 66, 862-865; Gupta P., et. al., 1999 Protein Expr. Purif. 16, 369-376; Kumar P., et. al. 2001, Infect. Immun., 69, 6532-6536). The vector contains T5 promoter and a 6X -Histidine tag, which allows convenient purification of the recombinant proteins (FIG. 1).

Conditions for overexpression of the said genes using the above mentioned recombinant plasmids, from E. coli strains have been optimized by the inventors (Chauhan V., et. al., 2001, Biochem. Biophys. Res. Commun., 283, 308-315).

Using the above mentioned recombinant plasmid, inventors of the present process, introduced mutations in the said genes to make the expressed recombinant proteins defective in their biological function, thereby rendering them non-toxic. The invention involves the expression and purification of the said mutant proteins from E. coli strains. It further involves full characterization of the purified mutant proteins to pinpoint the defect that renders them non-toxic.

Mutations Introduced in Protective Antigen as Part of the Invention

-   1. Mutations that make PA defective in binding to LF/EF. The     inventors introduced series of mutations in the 1^(st) domain of PA.     Among the mutations introduced, the mutations at residues 202, 203,     205, 207 and 205+226+236 were found to be defective in binding to     LF. -   2. Mutations that make PA defective in oligomerization. The authors     of this invention introduced mutations in the 3^(rd) domain of PA.     The mutation at the residues 552, 574, 552+554, 562+574 566+574,     552+554+562+566+574 resulted in mutant proteins that were defective     in oligomerization. -   3. Mutations that make PA translocation-defective. Inventors have     introduced mutations at residues 425 and 427 of loop 4 of domain 2.     These mutations impaired the translocation-ability of PA. -   4. Mutations that make PA defective in insertion/translocation.     Authors have discovered that when mutations are introduced at the     residues 346, 352 and 346+350+352 in loop 3 of domain 2, PA becomes     biologically inactive. The mutant proteins were able to bind to the     cell-surface receptors, get proteolytically activated to form     oligomers and bind to LF. The biological inactivity of these mutant     proteins may pertain to a defect in insertion/translocation.     Mutations Introduced in Lethal Factor as Part of the Invention -   Mutations that make LF defective in binding to PA. The inventors of     this process have introduced mutations in the 1^(st) domain of LF.     They found that mutation at residues 148, 149, 151, 153, 187, 190     and 187+188+189+190 impaired the binding of LF to PA.     Mutations Introduced in Edema Factor as Part of the Invention -   Mutations that make EF defective in binding to PA. The inventors of     this process have introduced series of mutations in the 1^(st) 250     residues of EF. It was found that mutation at residues 137, 138, 140     and 142 drastically impaired the binding of EF to PA.

After the expression and purification of the mutant proteins the proteins were evaluated for their biological activity.

Inventors have found that the above-mentioned mutants of PA when added along with wild-type LF, were nontoxic to J774A.1 cells. Likewise mutants of LF when added along with wild-type PA were non-toxic to J774A.1 cells. Similarly, mutants of EF when added along with wild-type PA were unable to produce cAMP-toxicity in CHO cells (Table 2).

The purified mutant protein was analyzed for their biological activity by assaying:

-   -   Ability of PA to bind to cell surface receptors,     -   Ability of PA to bind to LF or EF,     -   Ability of PA to oligomerize,     -   Membrane insertion ability of PA oligomer,     -   Ability of PA to translocate LF or EF to the cytosol,     -   Ability of lethal toxin to kill macrophage cell lines like         RAW264.7 and J774A.1     -   Ability of edema toxin to elongate CHO cells.         Immunization Studies

Protective antigen, as the name suggests is a highly immunogenic protein. In fact it is a necessary component of the vaccine against anthrax. Immunization with wild-type recombinant PA elicits high anti-PA titers and provides protection against anthrax lethal challenge in guinea pigs. It was further observed that mutant PA was as immunogenic as the wild-type PA and could easily substitute the wild-type PA in vaccine (Singh et. al. 1998, Infect. Immun. 66, 3447-3448). Immunization studies also indicate a significant contribution of LF/EF to immunoprotection. On basis of these results the inventors have developed a recombinant vaccine against anthrax, which comprises mutants of all the three anthrax toxin components.

The anthrax toxin based recombinant vaccine developed by the inventors has the following advantages:

-   1. The process described here does not involve handling of B.     anthracis cultures (at any stage). This process is therefore safe,     cost-effective and does not require the sophisticated containment     facilities. -   2. The vaccine developed by the inventors has well-defined     composition and will therefore not have any batch to batch     variation. -   3. The invention described here utilizes purified mutant anthrax     toxin protein. As a result, this second-generation anthrax vaccine     will not be reactogenic and will not cause any side-effects unlike     the previous vaccine. -   4. Additionally, this invention comprises of non-toxic mutant     proteins, which when administered (either alone or in combination)     do not cause any toxigenicity or pathogenicity as associated with     the currently used vaccine. -   5. The invention described here is therefore safe and suitable for     animal/human use.     Details of the Experimental Procedures

Site-directed Mutagenesis of Anthrax Toxin Proteins

To introduce the desired mutations in the anthrax toxin proteins, complementary mutagenic primers were used (refer Table 1) to amplify the wild type anthrax toxin genes (for PA or LF or EF). High fidelity Pfu DNA polymerase was used for the PCR reaction. Entire lengths of both the strands of the plasmid DNA were amplified in a linear fashion during several rounds of thermal cycling, generating a mutant plasmid with staggered nicks on the opposite strands (FIG. 2). The amplification was checked by agarose gel electrophoresis of the PCR product. The product of the amplification was treated with DpnI that specifically cleaves fully methylated G^(me6) ATC sequences. The digestion reaction was carried out in 20 μl reaction volume with 10 ng of the amplified product, 2 μl of 10X DpnI reaction buffer and 0.1 U of DpnI. After DpnI digestion, DpnI resistant molecules that are rich in desired mutants were recovered by transformation of the DNA into the appropriate E. coli strain. The mutations were confirmed by sequencing of the above constructs using Perlcin Elmer cycle DNA sequencing kit.

Expression and Purification of the Mutant Anthrax Toxin Proteins

The confined constructs were transformed into E. coli expression strains expressing T5 RNA polymerase. Transformed cells were grown in Luria broth medium (LB) containing 100 μg/ml of ampicillin and 25 μg/ml of kanamycin, at 37° C., to an OD₆₀₀ of 0.8. Induction vas then done with 0.5 mM IPTG and the incubation was continued at 37° C. for 3 to 4 hours. Cells were then harvested by centrifugation at 6000 rpm for 10 minutes. The cells then lysed. The protein profile was analysed by SDS-PAGE and western blotting. The mutant PA proteins were purified using Ni-NTA metal-chelate affinity chromatography and other chromatographic techniques (Kumar P., et. al. 2001, Infect. Immun., 69, 6532-6536; Gupta P., et. al., 1998, Infect. Immun., 66, 862-865; Gupta P., et. al., 1999 Protein Expr. Purif. 16, 369-376). The purified mutant proteins were analysed by SDS-PAGE and western blotting and were estimated using Bradford's method. For storage the purified proteins were dialysed against 50 mM HEPES and stored as aliquotes at −70° C.

Cell Culture

Macrophage like cell line J774A.1 was maintained in RPMI 1640 medium containing 10% heat inactivated FCS, 25 mM HEPES, 100U/ml penicillin and 200 μg/ml streptomycin in a humidified 5% CO₂ environment at 37° C.

CHO cells were maintained in EMEM medium containing 10% heat inactivated FCS, 25 mM HEPES, 100U/ml penicillin and 200 μg/ml streptomycin in a humidified 5% CO₂ environment at 37° C.

To study the biological activity of the wild-type PA or its mutant proteins, varying concentrations of these proteins were added along with LF (1 μg/ml) to J774A.1 cells plated in 96-wells plates. Incubation was allowed for 3 hrs. at 37° C. and then cell viability (Bhatnagar et. al. 1989, Infect. Immun., 57, 2107-2114) was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) dye (Bhatnagar R., et. al., 1999, Cell Signal., 11, 111-116). MT-T dissolved in RPMI was added to each well at a final concentration of 0.5 mg/ml and incubated for another 45 min at 37° C. to allow uptake and oxidation of the dye by viable cells. The medium was replaced by 0.5% (w/v) sodium dodecyl sulphate (SDS), 25 mM HCl in 90% isopropyl alcohol and the plate vortexed. The absorption was read at 540 nm using microplate reader (BIORAD).

Similarly, to study the biological activity of wild-type LF or its mutant proteins, varying concentrations of these proteins were added along with PA (1 μg/ml) to J774A.1 cells plated in 96-wells plates. Incubation was allowed for 3 hrs. at 37° C. and then cell viability as determined using MTT dye, as detailed above.

To study the biological activity of wild-type EF or its mutant proteins, varying concentrations of these proteins were added along with PA (1 μg/ml) to CHO cells that were plated in 96-wells plates. Incubation was allowed for 3 hrs. at 37° C. and then the cells were microscopically examined for elongation. Rise in intracellular cAMP levels of the cells upon toxin treatment was determined (Kumar P., et. al., 2001, Infect. Immun., 69, 6532-6536) with cAMP EIA kit of Amersham Pharmacia.

Further experiments were then done to understand how mutations were affecting the biological activity of the anthrax toxin mutant proteins.

Binding of PA to Cell Surface Receptors

J774A.1 cells were allowed to grow to confluence in 24-well plates before incubating with 1 μg/ml of wild-type PA or its mutant protein at 4° C. for 2 hrs. The cells were then washed with cold RPMI, dissolved in SDS lysis buffer and subjected to SDS-PAGE for electroblotting. The blot was developed with anti-PA antibodies to study the binding of wild-type PA or its mutant protein with the cell surface receptors

Proteolytic Cleavage of PA and Mutant Proteins in Solution

Wild-type PA and its mutant proteins were tested for susceptibility to cleavage by trypsin. The proteins (1.0 mg/ml) were incubated with 1 μg/ml of trypsin for 30 minutes at room temperature in 25 mM HEPES, 1 mM CaCl₂, 0.5 mM EDTA pH 7.5. The digestion reaction was stopped by adding PMSF to a concentration of 1 mM. For SDS-PAGE, the samples were boiled in SDS sample buffer for 5 minutes and resolved on 12% SDS-PAGE.

Binding of PA to LF on the Surface of Cells

J774A. 1 cells were washed twice with RPMI and then incubated with 1 μg/ml of wild-type PA or its mutant protein at 4° C. for 3 hrs. The cells were then washed with cold RPMI to remove unbound protein. The cells were further incubated with LF (1.0 μg/ml) for 3 hours and then washed with cold RPMI to remove unbound LF. The cells were dissolved in SDS lysis buffer and subjected to SDS-PAGE for electroblotting. The blot was developed with anti-LF antibodies to study the binding of the wild-type PA or its mutant protein to LF.

Oligomerization of PA in Solution

PA upon proteolytic cleavage oligomerises to form heptamers. To study the ability of the wild-type PA and its mutant proteins to form oligomers, the proteins (1 mg/ml) were digested with trypsin for 30 minutes at 25° C. The samples were brought to pH 5.0 by addition of 1M Tris pH5.0 to a final concentration of 100 mM. and were boiled for 5 minutes in SDS sample buffer (0.0625M Tris-Cl, 1.25% SDS, 2.5%, β-mercaptoethanol and 5% glycerol, pH6.8) before loading on a 3-12% gradient gel. Silver staining was done to detect the formation of oligomers.

Binding of LF/EF to Cell-surface Bound PA.

J774A.1 cells were washed with cold RPMI and then incubated with 1 μg/ml of wild-type PA at 4° C. for 3 hrs. The cells were washed again with cold RPMI to remove unbound protein. Wild-type LF/EF or the mutant proteins (1.0 μg/ml) were then added and incubation was continued for 3 hours. The cells were then washed with cold RPMI to remove unbound LF/EF. Later, the cells were dissolved in SDS lysis buffer and subjected to SDS-PAGE for electroblotting. The blot was developed with anti-LF/EF antibodies to study the binding of LF/EF to cell-surface bound.

RESIDUE CHANGE PRIMERS DOMAIN DEFECT PA mutants: Phe202 To alanine 5′CTTTTCATGAATATTAGAAATCCATGCTGAAAG I Defective in (SEQ ID NO: 1) binding to Lethal factor Leu203 To alanine CTTTTCATGAATATTAGAAATCCATGGTGAAGCAAAAGT I Defective in (SEQ ID NO: 2) binding to Lethal factor Pro205 To alanine CTTTTCATGAATATTAGAAATCCATGGTGAAAGAGCAGTTCT I Defective in (SEQ ID NO: 3) binding to Lethal factor Ile207 To alanine TTTGGTTAACCCTTTCTTTTCATGAATATTAGAAATCCATGGT I Defective in GAAAGAAAAGTTCTTTTATTTTTGACATCAACCGTATATCCTT binding to Lethal CTACCTCTAATGAATCAGCGATTCC (SEQ ID NO: 4) factor Pro205 + Trp226 + To alanine CTTTTCATGAATATTAGAAATCCATGGTGAAAGAGCAGTTCT  I Defective in Phe236 (SEQ ID NO: 5) and binding to Lethal GGATTTCTAATATTCATGAAAAGAAAGGATTAACCAAATATA factor AATCATCTCCTGAAAAAGCGAGCACGGCTTCTGATCCGTACA GTGATGCCGAAAAGGTT (SEQ ID NO: 6) Phe552 To alanine CAAGGGAAAGATATCACCGAATTTGATGCTAATTTCGATC III Oligomerization (SEQ ID NO: 7) defective Ile574 To alanine GAATTAAACGCGTCTAACGCATATACTG III Oligomerization (SEQ ID NO: 8) defective Phe552 + Phe554 To alanine ATTTTGAGATGTTTGTTGATCGGCATTAGCATCAAATTC III Oligomerization (SEQ ID NO: 9) defective Ile562 + Ile574 To alanine CAGTATATGCGTTAGACGCGTTTAATTCCGCTTAACTGATTCT III Oligomerization TGGCATTTTGAGATG (SEQ ID NO: 10) defective Leu566 + Ile574 To alanine ATCAGGCAGCGGAATTAAACGCGTCTAACGCATATACTG III Oligomerization (SEQ ID NO: 11) defective Phe552 + Phe554 + To alanine CAGTATATGCGTTAGACGCGTTTAATTCCGCTGCCTGATTCTT III Oligomerization Ile562 + Leu566 + GGCATTTTGAGATG (SEQ ID NO: 12 )and defective Ile574 ATTTTGAGATGTTTGTTGATCGGCATTAGCATCAAATT (SEQ ID NO: 13) Phe427 To alanine GTAATTGGAGTAGAACTGGCATCGTCTTGTGC II Translocation (SEQ ID NO: 14) defective Asp425 Residue GTAATTGGAGTAGAACTGAAATCTTGTTCATTTAATGCG II Translocation deleted (SEQ ID NO: 15) defective Phe427 Residue GCACAAGACGATAGTTCTACTCCAATTAC II Translocation deleted (SEQ ID NO: 16) defective Trp346 To alanine CGGTCGCAATTGATCATTCACTATCTCTAGCAGGGGAAAGAA II Membrane CTGCGGCTGAAACAATG (SEQ ID NO: 17) insertion/ translocation defective Leu352 To alanine CGGTCGCAATTGATCATTCACTATCTCTAGCAGGGGAAAGAA II Membrane CTTGGGCTGAAACAATGGGTGCAAATACCGCTGAT insertion/ (SEQ ID NO: 18) translocation defective Trp346, Met350 To alanine CGGTCGCAATTGATCATTCACTATCTCTAGCAGGGGAAAGAA II Membrane and Leu352 CTGCGGCTGAAACAGCGGGTGCAAATACCGCTGAT insertion/ (SEQ ID NO: 19) translocation defective LF mutants: Tyr148 To alanine GTAGAAGGTACCGAAAAGGCACTGAACGTTGCTTAT I Defective in (SEQ ID NO: 20) binding to Protective Antigen Tyr149 To alanine GTAGAAGGTACCGAAAAGGCACTGAACGTTTATGCTGAA I Defective in (SEQ ID NO: 21) binding to Protective Antigen Ile151 To alanine GTAGAAGGTACCGAAAAGGCACTGAACGTTTATGAAGCAGGT I Defective in (SEQ ID NO: 22) binding to Protective Antigen Lys153 To alanine GTAGAAGGTACCGAAAAGGCACTGAACGTTTATGAAATAGGT I Defective in GCAATA (SEQ ID NO: 23) binding to Protective Antigen Asp187 To alanine TGTGGGATGTTCCTTAAGCTGATTAGTAAATAAAAGAGCTTGT I Defective in TCATCTGA (SEQ ID NO: 24) binding to Protective Antigen Phe190 To alanine TGTGGGATGTTCCTTAAGCTGATTAGTAGCTAAAAGATCTTG I Defective in (SEQ ID NO: 25) binding to Protective Antigen Asp187, Leu188, To alanine TGTGGGATGTTCCTTAAGCTGATTAGTAGCTGGAGCAGCTTGT I Defective in Leu189, Phe190 TCATCTGA (SEQ ID NO: 26) binding to Protective Antigen EF mutants: Tyr137 To alanine CCTTACTTATGATATCAAGAGAAATCCCC TTT CC AAT TTC Defective in binding AGC ATA TAC TTC TTT ACT TTG TTC AC to Protective Antigen (SEQ ID NO: 27) Tyr138 To alanine CCTTACTTATGATATCAAGAGAAATCCCC TTT CC AAT TTC Defective in binding ATA AGCTAC TTC TTT ACT TTG TTC AC to Protective Antigen (SEQ ID NO: 28) Ile140 To alanine CCTTACTTATGATATCAAGAGAAATCCCC TTT CCAGCTTC Defective in binding ATA ATATAC TTC TTT ACT TTG TTC AC to Protective Antigen (SEQ ID NO: 29) Lys142 To alanine CCTTACTTATGATATCAAGAGAAATCCCC GCT CC AAT TTC Defective in binding ATA ATATAC TTC TTT ACT TTG TTC AC to Protective Antigen (SEQ ID NO: 30)

TABLE 2 CHARACTERISTICS OF MUTANTS OLIGOMER LF/EF MUTATION RECEPTOR TRYPSIN FOR- BIND- TOX- IN PA BINDING NICKING MATION ING ICITY Phe202Ala + + + − − Leu203Ala + + + − − Pro205Ala + + + − − Ile207Ala + + + − − Pro205Ala + + + + − − Trp226Ala + Phe236Ala Phe552Ala + + − − − Ile574Ala + + − − − Phe552Ala + + + − − − Phe554Ala Ile562Ala + + + − − − Ile574Ala Leu566Ala + + + − − − Ile574Ala Phe552Ala + + + − − − Phe554Ala + Ile562Ala + Leu566ala + Ile574Ala Phe427Ala + + + + − Asp425del + + + + − Phe427del + + + + − Trp346Ala + + + + − Leu352Ala + + + − Trp346Ala + + + + + − Met350Ala + Leu352Ala MUTATION IN LF BINDING TO PA TOXICITY Tyr148Ala − − Tyr149Ala − − Ile151Ala − − Lys153Ala − − Asp187Ala − − Phe190Ala − − Asp187Ala + Leu188Ala + − − Phe190Leu189Ala MUTATION IN EF BINDING TO PA TOXICITY Tyr137Ala − − Tyr138Ala − − Ile140Ala − − Lys142Ala − − 

1. A recombinant DNA construct for expressing mutagenized toxin protein of Bacillus anthracis comprising: A) an expression vector, and B) a DNA fragment comprising a gene encoding mutant type Protective Antigen (PA), mutant type Lethal Factor (LF) or mutant type Edema Factor (EF), or a combination thereof wherein the DNA fragment comprises a non-native sequence that results from amplifying at least one of (i) a gene encoding native Protective Antigen (PA) with a primer selected from the group consisting of SEQ ID NO: 1, 2, 3, 4, 5, and 6; (ii) amplifying a gene encoding native Lethal Factor (LF) with a primer selected from the group consisting of SEQ ID NO: 20, 21, 22, 23, 24, 25 and 26; and (iii) amplifying a gene encoding native Edema Factor (EF) with a primer selected from the group consisting of SEQ ID NO: 27, 28, 29 and
 30. 2. A recombinant DNA construct as claimed in claim 1, wherein said expression vector is a prokaryotic vector.
 3. A recombinant DNA construct as claimed in claim 2, wherein said prokaryotic expression vector is pQE30.
 4. A recombinant DNA construct as claimed in claim 3, wherein said prokaryotic expression vector contains T5 promoter and 6X histidine tag.
 5. A mutagenized PA, LF or EF anthrax toxin protein of Bacillus anthracis, expressed by the recombinant DNA construct of claim 1, wherein the protein possesses immunogenic properties.
 6. A method of producing a mutagenized PA, LF or EF anthrax toxin protein, wherein the method comprises the following steps: a) producing a mutant gene encoding a mutant toxin protein PA, LF or EF by amplifying a native PA, LF or EF template in a polymerase chain reaction using a primer selected from the group consisting of SEQ ID NO: 1, 2, 3, 4, 5, 6, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 and 30 to form mutant polymerase chain reaction products, b) treating the mutant polymerase chain reaction products along with the native template from step a) with an enzyme to cleave the native template of said polymerase chain reaction products to form mutant products, c) transforming a prokaryotic host with said mutant products of step b to cause the prokaryotic host to express the mutagenized PA, LF or EF anthrax toxin protein, and d) purifying the expressed mutagenized PA, LF or EF anthrax toxin protein.
 7. A method as claimed in claim 6, wherein the prokaryotic host is an E. coli strain.
 8. A method as claimed in claim 6, wherein said enzyme is DpnI enzyme that specifically cleaves fully methylated target sequence G^(me6) ATC.
 9. A method as claimed in claim 6, wherein saidm—mutant gene is cloned in a pQE30 expression vector under the control of T5 promoter and 6X histidine tag fusion.
 10. A method as claimed in claim 6, wherein said purification is carried out using Ni-NTA affinity chromatography.
 11. An anthrax vaccine comprising a mutant toxin protein of Bacillus anthracis encoded by the recombinant DNA construct of claim 1, said vaccine comprising a mutant toxin protein PA, LF or EF, or a combination thereof.
 12. An anthrax vaccine comprising one or more mutagenized anthrax toxin proteins of claim 5, in combination with a toxin protein of Bacillus anthracis of wild-type PA or wild-type LF or wild-type EF.
 13. A mutagenized PA anthrax toxin protein produced by the method of claim
 6. 14. A mutagenized LF anthrax toxin protein produced by the method of claim
 6. 15. A mutagenized EF anthrax toxin protein produced by the method of claim
 6. 16. A recombinant DNA construct as claimed in claim 1, wherein the DNA fragment comprises a non-native sequence that results from amplifying a gene encoding native Protective Antigen (PA).
 17. A recombinant DNA construct as claimed in claim 1, wherein the DNA fragment comprises a non-native sequence that results from amplifying a gene encoding native Lethal Factor (LF).
 18. A recombinant DNA construct as claimed in claim 1, wherein the DNA fragment comprises a non-native sequence that results from amplifying a gene encoding native Edema Factor (EF). 